Blood Types

If a doctor talks about your blood type, usually he or she is referring to two things: your type in the ABO system and Rhesus (Rh) factor. Human blood type is determined by antigens on the red blood cells. An antigen is a structure on the cell surface that causes a human immune response reacts to if the structure is foreign to the person’s body. Consequently, blood type match is of crucial importance. The donor’s blood type is identified at the Blood Centre, and the patient’s blood type is determined before transfusion. Here is everything you need to know!

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How do blood types work?

Logically, you inherit your blood type from a mix of your parents’ genes. There are eight main blood types, organized through two combined systems. These systems are ABO (blood types A, B, AB or O) and Rh type or group (positive or negative). Therefore, your blood type is a combination of these two systems. For instance, the most common blood type is O positive and the least common is AB negative.

The most important blood group system is ABO, in which your blood is classified as A, B, O or AB. This is determined by two antigens on the red blood cells:

A — red blood cells have only the A antigen

B — red blood cells have only the B antigen

AB — red blood cells have the A and B antigens

O — neither A nor B antigen

If a person has A, B or O blood type, their plasma contains antibodies that destroy the antigens that the person doesn’t have. Here is a good example: If you have blood type A, it’s important that you don’t receive a B type transfusion, as you have antibodies that will destroy B antigens. If you have blood type O, you have antibodies that will fight the A and B antigens.

If a person has blood type AB, they don’t have such antibodies, and they can accept transfusions from all other blood types. Thus, AB blood type people can be termed universal patients.

However, O Rh negative donors can be called universal donors, as red blood cells from such donors can be used for transfusions for all patients.

The Rh factor

Each blood type is also grouped by its Rhesus factor, or Rh factor. Blood is either Rh positive (Rh+) or Rh negative (Rh-). Statistics says that about 85% of Americans have Rh+ blood. Rhesus refers to another type of antigen, or protein, on the surface of red blood cells. The name Rhesus comes from Rhesus monkeys, in which the protein was first discovered.

Blood Transfusions

A blood transfusion is the transfer of blood from one person to another. Blood that is lost through an injury, an illness or a surgery can be replaced through transfusion. Aside from transferring blood as a whole, part of blood, such as red blood cells, platelets or plasma can also be transferred to individuals. Donor blood is always tested for HIV, hepatitis, syphilis, West Nile virus and other diseases before every transfusion. There are more than 9.5 million blood donors in the United States and an estimated 5 million patients who receive blood annually, resulting in a total of 14.6 million transfusions per year.

What is the Rarest Blood Type?

According to the American Red Cross the rarest is AB(-), present in 1% of the Caucasians, in African Americans it is even rarer. B(-) and O(-) are also very rare, each accounting for less than 5% of the world’s population. Some people with rare blood types bank their own blood in advance of surgical procedures to ensure that blood is available to them which is always a smart move.

Blood types are inherited. Like eye color, blood type is passed genetically from your parents. Whether your blood group is type A, B, AB or O is based on the blood types of your mother and father.


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